Transparent electrically-conductive layers (TCL) of metal oxides such as indium tin oxide (ITO), antimony doped tin oxide, and cadmium stannate (cadmium tin oxide) are commonly used in the manufacture of electrooptical display devices such as liquid crystal display devices (LCDs), electroluminescent display devices, photocells, solid-state image sensors or electrochromic windows.
Devices such as flat panel displays typically contain a substrate provided with an indium tin oxide (ITO) layer as a transparent electrode. The coating of ITO is carried out by vacuum sputtering methods, which involve high substrate temperature conditions up to 250° C., and therefore, glass substrates are generally used. The high cost of the fabrication methods and the low flexibility of such electrodes, due to the brittleness of the inorganic ITO layer as well as the glass substrate, limit the range of potential applications. As a result, there is a growing interest in making all-organic devices, comprising plastic resins as a flexible substrate and organic electroconductive polymer layers as an electrode. Such plastic electronics allow low cost devices with new properties. Flexible plastic substrates can be provided with an electroconductive polymer layer by continuous hopper or roller coating methods (compared to batch process such as sputtering) and the resulting organic electrodes enable the “roll to roll” fabrication of electronic devices which are more flexible, lower cost, and lower weight.
Intrinsically conductive polymers have recently received attention from various industries because of their electronic conductivity. Although many of these polymers are highly colored and are less suited for TCL applications, some of these intrinsically conductive polymers, such as substituted or unsubstituted pyrrole-containing polymers (as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654), substituted or unsubstituted thiophene-containing polymers (as mentioned in U.S. Pat. Nos. 5,300,575, 5,312,681, 5,354,613, 5,370,981, 5,372,924, 5,391,472, 5,403,467, 5,443,944, 5,575,898, 4,987,042, and 4,731,408) and substituted or unsubstituted aniline-containing polymers (as mentioned in U.S. Pat. Nos. 5,716,550, 5,093,439, and 4,070,189) are transparent and not prohibitively colored, at least when coated in thin layers at moderate coverage. Because of their electronic conductivity instead of ionic conductivity, these polymers are conducting even at low humidity.
EP-A-440 957 describes a method for preparing polythiophene in an aqueous mixture by oxidative polymerization in the presence of a polyanion as a doping agent. In EP-A-686 662 it has been disclosed that highly conductive layers of polythiophene, coated from an aqueous coating solution, could be made by the addition of a di- or polyhydroxy and/or a carbonic acid, amide or lactam group containing compound in the coating solution of the polythiophene. Coated layers of organic electroconductive polymers can be patterned into electrode arrays using different methods. The known wet-etching microlithography technique is described in WO97/18944 and U.S. Pat. No. 5,976,274 wherein a positive or negative photoresist is applied on top of a coated layer of an organic electroconductive polymer, and after the steps of selectively exposing the photoresist to UV light, developing the photoresist, etching the electroconductive polymer layer and finally stripping the non-developed photoresist, a patterned layer is obtained. In U.S. Pat. No. 5,561,030 a similar method is used to form the pattern except that the pattern is formed in a continuous layer of prepolymer which is not yet conductive and that after washing the mask away the remaining prepolymer is rendered conductive by oxidation. Such methods that involve conventional lithographic techniques are cumbersome as they involve many steps and require the use of hazardous chemicals.
EP-A-615 256 describes a method to produce a pattern of a conductive polymer on a substrate that involves coating and drying a composition containing 3,4-ethylenedioxythiophene monomer, an oxidation agent, and a base; exposing the dried layer to UV radiation through a mask; and then heating. The UV exposed areas of the coating comprise non-conductive polymer and the unexposed areas comprise conductive polymer. The formation of a conductive polymer pattern in accordance with this method does not require the coating and patterning of a separate photoresist layer.
U.S. Pat. No. 6,045,977 describes a process for patterning conductive polyaniline layers containing a photobase generator. UV exposure of such layers produces a base that reduces the conductivity in the exposed areas.
EP-A-1 054 414 describes a method to pattern a conductive polymer layer by printing an electrode pattern onto said conductive polymer layer using a printing solution containing an oxidant selected from the group ClO−, BrO−MnO4−, Cr2O7−2, S2O8−2, and H2O2. The areas of the conductive layer expos the oxidant solution are rendered nonconductive.
Research Disclosure, November 1998, page 1473 (disclosure no. 41548) describes various means to form patterns in conducting polymer, including photoablation wherein the selected areas are removed from the substrate by laser irradiation. Such photoablation processes are convenient, dry, one-step methods but the generation of debris may require a wet cleaning step and may contaminate the optics and mechanics of the laser device. Prior art methods involving removal of the electroconductive polymer to form the electrode pattern also induce a difference of the optical density between electroconductive and non-conductive areas of the patterned surface, which should be avoided.
Methods of patterning organic electroconductive polymer layers by image-wise heating by means of a laser have been disclosed in EP 1 079 397 A1. That method induces about a 10 to 1000 fold decrease in resistivity without substantially ablating or destroying the layer.
As indicated herein above, the art discloses a wide variety of electrically conductive thin film compositions. However, there is still a critical need in the art for a patterned conductive thin film structures. In addition to providing superior electrode performance, these thin film conductive layers also must be patternable, must resist the effects of humidity change, and be manufacturable at a reasonable cost.
It is toward the objective of providing such improved electrically conductive, patternable, preferably web coatable, thin films that more effectively meet the diverse commercial needs than those of the prior art, that the present invention is directed.
The thin film electrode layers of this invention are patterned by electrographic development process. The electrographic marking toner particles comprising conductivity enhancing or conductivity degrading agent(s) are employed for electrographic patterning of the electrode layer. The final pattern is “fixed” by means of pressure and (or) heat fixing step, whereupon the conductivity modifier material and other substances incorporated in the marking toner particle interact with, the electrically conductive polymer material and localized alteration i.e. enhancement or degradation of conductivity occurs, whereupon an electrode pattern is developed.
Electrographic imaging and development processes have been extensively described in both the patent and technical literature. Such electrographic imaging and development processes include the processes of electrophotography, electrophoretic migration imaging and modulated electrostatic printing. A typical electrophotographic process employs a photoconductive element comprising a coating of a photoconductive insulating material on a conductive support. The element is given a uniform charge in the dark and then is exposed to an image pattern of activating electromagnetic radiation such as white light or X-rays. The charge on the photoconductive element is dissipated in the illuminated areas to form an electrostatic charge pattern which is then developed by contact with a developer composition comprising a carrier and an electrographic toner marking material. The marking particles deposit on the surface bearing the electrostatic charge pattern, in accordance with either the charge pattern or the discharge patterns as desired.
In a typical electrophoretic migration imaging process, an imaging composition comprising electrostatic charge-bearing photoconductive particles, i.e. electrically photosensitive particles, is positioned between two spaced electrodes, one of which may be transparent. To achieve image formation in this process, the electrically photosensitive particles positioned between the two spaced electrodes are subjected to the influence of an electric field and exposed to a pattern of activating radiation. As a result, the electrically photosensitive particles are caused to migrate electrophoretically to the surface of one or the other of the spaced electrodes upon which an image of the pattern of activating radiation is defined by the electrically photosensitive particles.
Typically, a negative image of the pattern is formed on one electrode, and a positive image of the pattern is formed on the opposite electrode. One method of modulated electrostatic printing referred to in the patent literature comprises generating an ion stream in the direction of a print receiving medium, modulating the cross-sectional density flow of ions in the stream in accordance with a pattern to be reproduced, and introducing a cloud of substantially uncharged marking particles adjacent to print receiving medium whereby the modulated ion stream selectively collides with and induces charges on the marking particles in the cloud. The marking particles are then deposited on the printing receiving medium in accordance with the pattern being reproduced. The direction of the ion stream flow is determined by an electrical field. The ion stream is modulated in accordance with the charge pattern by a grid having computer addressed electronic gates.